1. Field of the Invention
The present invention relates to a technology for controlling an acousto-optic tunable filter (AOTF) having a temperature characteristic. More particularly, the present invention relates to a technology for causing the AOTF to selectively output a signal of a predetermined wavelength by correcting the frequency deviation, which is due to the temperature characteristic, of a radio-frequency (RF) signal to be input to the AOTF.
2. Description of the Related Art
With a purpose of building a multimedia network, an optical communication device that enables long distance transmission of large-amount data has been demanded. To achieve increased data-transmission capacity, research and development on a wavelength-division multiplexing (WDM) has been carried out because the WDM has advantage in which a broadband property or a large capacity property of an optical fiber can be efficiently utilized.
In the optical communication network, a function of transmitting, dropping and adding an optical signal at each point on the network as necessary, an optical routing function for selecting an optical transmission path, and a cross-connect function are necessary. For this reason, an optical add/drop multiplexer (OADM) that transmits, drops, and adds an optical signal has been developed. The OADM includes a fixed-wavelength type and a selectable-wavelength type. The fixed-wavelength type can add/drop only an optical signal having a fixed wavelength. The selectable-wavelength type can add/drop an optical signal having an arbitrary wavelength.
Conventionally, an acousto-optic tunable filter (AOTF) is used to realize an OADM of the selectable-wavelength type. The AOTF acts as to extract only a light having a selected wavelength. Therefore, unlike a fiber grating in which a selected wavelength is fixed, it is possible to arbitrarily select a wavelength. Since the AOTF functions as a tunable wavelength-selecting filter, the AOTF can be applied to a tunable wavelength-selecting filter for a tributary station that adds/drops an optical signal between terminal stations. With such reasons, the OADM using the AOTF is being developed (see, for example, Japanese Patent Application Laid-Open No. H11-218790).
In the AOTF, a radio frequency signal (hereinafter, “RF signal”) having a frequency band of 160 megahertzs (MHz) to 180 MHz applied to the AOTF functions as a control signal, and the AOTF outputs an optical signal according to the frequency. However, since the AOTF has temperature-dependent properties, even if an identical RF signal is applied to the AOTF, a wavelength of an optical signal to be output varies depending on temperature. Therefore, an AOTF subsystem to obtain an RF signal to output a desirable wavelength based a reference light having a predetermined wavelength output from a reference light source has been proposed.
However, as described above, with the AOTF, even if an RF signal has an identical frequency, the wavelength of the optical signal to be output varies if ambient temperature changes. Thus, a wavelength that can be obtained also changes, specifically, the wavelength obtained shifts 0.8 nanometer (nm) as the ambient temperature changes each 1° C. This amount of wavelength shift reaches an amount of interval between the selected wavelength and adjacent wavelengths.
In a method of selecting a wavelength in the AOTF subsystem described above, preparing a reference light having the shortest wavelength and a reference light having the longest wavelength, and by tracking the reference wavelengths, a desirable frequency of the RF signal is calculated based on number of wavelengths and difference between RF frequencies of the reference lights. However, in this method, it is necessary to prepare two reference light sources. As a result, cost increases.
Moreover, there is another problem if wavelength selection is to be performed only with a single reference light source, that is, temperature-dependent frequency-pulling effect. FIG. 7 is a schematic for illustrating the temperature-dependent frequency-pulling effect of an AOTF of a dropping type.
A chart 701 illustrates a wavelength arrangement for output signals λ1 to λn and reference lights λref1 and λref2 when a WDM signal is input to an input port and the reference lights are input to ports for a reference light of the AOTF. As shown in the chart 701, the WDM transmission signal is formed with optical signals having a frequency interval (grid) of 100 gigahertz (GHz). For example, in a wavelength light having a C band (1530 nm to 1565 nm), 32 optical signals are multiplexed. The reference light λref1 has a wavelength keeping the wavelength interval of 100 GHz from the optical signal λ1 having the shortest wavelength. Similarly, the reference light λref2 has a wavelength keeping the wavelength interval of 100 GHz from the optical signal λn having the longest wavelength.
A chart 702 illustrates a wavelength arrangement for the optical signals λ1 to λn and the reference lights λref1 and λref2 when the optical signal λ2 is output from one of output ports of the AOTF when the temperature of the AOTF is 25° C. A solid line indicates the optical light being output and broken lines indicate optical lights not being output and the reference lights.
A chart 703 illustrates a frequency arrangement for RF signals F1 to Fn and Fref1 and Fref2 to output the optical signals and the reference lights input to the AOTF. A solid line shown in the chart 703 indicates the RF signal F2 that is applied to the AOTF when the optical signal λ2 shown in the chart 702 is to be output. The RF signals F1 to Fn are RF signals to output the optical signals λ1 to λn when temperature of the AOTF is 25° C. Since the optical signals are arranged at regular intervals, the RF signals are also arranged at regular intervals of Δf1. Each of the optical signals shown in the chart 702 with the broken lines are output by applying the RF signal corresponding to each of the optical signals.
However, when the temperature of the AOTF changes, relationship between a frequency of the RF signal to be applied and a wavelength of the optical signal to be output also changes. A chart 704 illustrates frequencies of RF signals F1′ to Fn′ and Fref1′ and Fref2′ when the output signal λ2 is output from one of the output ports when the temperature of the AOTF is 45° C. An RF signal F2′ shown with a solid line is the RF signal to output the optical signal λ2. The RF signals F1′ to Fn′ and Fref1′ and Fref2′ shown with broken lines are RF signals to output each of the optical signals and the reference lights. The RF signals F1′ to Fn′ and the Fref1′ and Fref2′ are arranged at regular intervals of Δf2 to output the optical signals arranged at the regular interval of 100 GHz. The interval Δf2 is smaller than the interval Δf1, which is an interval at which the RF signals are arranged when the temperature of the AOTF is 25° C.
Thus, when the temperature increases from 25° C. to 45° C., a frequency interval of the RF signals to output an optical signal having an identical frequency changes from Δf1 to Δf2 (Δf2<Δf1). Such a phenomenon is the temperature-dependent frequency-pulling effect. There is a problem caused by the temperature-dependent frequency-pulling effect when the wavelength selection is to be achieved only with a single reference-wavelength light source.